NONLETHAL IMPACTS OF PREDATOR INVASION - FACULTATIVE SUPPRESSION OF GROWTH AND REPRODUCTION

We asked whether invasions by a predator in a patchy environment altered only the death rate of the prey, or whether there were also nonlethal effects, i.e., alterations in three other vital rates: net emigration, reproduction, and individual growth rates. Field studies documented the patch use of the guppy Poecilia reticulata and the killifish Rivulus hartii in pools of a second-order forest stream in Trinidad, before and after invasion by the piscivorous fish Hoplias malabaricus. Experiments revealed that the predator altered the within-pool use of space by Poecilia and Rivulus, and caused significant emigration of the prey from pools in which it was present. Further, intimidation by the predator in an experimental stream suppressed total egg production in Rivulus by approximately 50%, and created spatial patchiness (more eggs laid in safer pools) and temporal patchiness (pulses of eggs) in egg production. The presence of the predator also induced shifts to shallow riffle areas and significantly reduced the growth rate of adult but not juvenile Rivulus. In contrast to the familiar paradigm that increased predation rates result in compensatory increases in per capita reproductive rates and/or growth rates as the population is thinned, we found that the threat of predation suppressed rates of reproduction and growth in predator-occupied patches.